* update binary to the latest commit (2021-07-29) to fix #16222 and #16239
* add hotplug.d/iface file and update Makefile to install it
* use Cloudflare's and Google's bootstrap DNS if bootstrap DNS is missing
* minor improvements in append_bool function
* add append_counter function for verbosity setting
* add append_bootstrap function (and supporting functions) to parse/sanitize bootstrap setting
* move firewall array from 'main' instance to the first proxy instance
* delete useless 'main' instace

Signed-off-by: Stan Grishin <stangri@melmac.net>